A plurality of electric motors mechanically coupled to one another makes possible a means whereby increased power can be obtained from the motors by adding a further motor for example, but this has not been regarded as feasible with AC or DC motors (other than series wound DC motors) previously employed. For example if two shunt wound DC motors are coupled together and are fed with power, a minor difference in the impedance of the one motor will cause a corresponding difference in current flow through that motor, but if the motor were operating without being coupled to the other motor, the difference would be compensated for by speed, the back EMF automatically being adjusted by the speed so that the amount of power drawn by the motor is an almost linear function of the power being transmitted by the motor shaft. The slip speed of a squirrel cage or other AC motor will respond in a similar manner. With two motors coupled together, quite minor differences of impedance will result in unequal speeds for the same torque, or unequal torques for the same speed. Unfortunately the characteristics of electric motors are such that small impedance differences result in quite considerable differences in the power consumption for the same speed, and in extreme instances, there can be a circulation of current between the two motors.
One of the main objects of this invention is to provide control means whereby motors can be directly coupled mechanically in such a way that the combined torque output of all motors is the sum of the torques of the motors and the motors are substantially evenly loaded.
Another object of the invention is to provide improvements in the means whereby the motors are interconnected.